fix: delete manifest before blobs by default

Currently the conformance tests default to deleting manifests after
blobs, which leaves the manifest dangling. Furthermore, the same blob may be
referenced in many manifests.

AFAIK, no client does this.

So changing default behavior.

Signed-off-by: Ramkumar Chinchani <rchincha@cisco.com>

conformance/setup.go

@@ -310,7 +310,7 @@ func init() {
 	runContentDiscoverySetup = true
 	runContentManagementSetup = true
 	skipEmptyLayerTest = false
-	deleteManifestBeforeBlobs = false
+	deleteManifestBeforeBlobs = true
 
 	if os.Getenv(envVarTagName) != "" &&
 		os.Getenv(envVarManifestDigest) != "" &&
@@ -323,7 +323,9 @@ func init() {
 	}
 
 	skipEmptyLayerTest, _ = strconv.ParseBool(os.Getenv(envVarPushEmptyLayer))
-	deleteManifestBeforeBlobs, _ = strconv.ParseBool(os.Getenv(envVarDeleteManifestBeforeBlobs))
+	if v, ok := os.LookupEnv(envVarDeleteManifestBeforeBlobs); ok {
+		deleteManifestBeforeBlobs, _ = strconv.ParseBool(v)
+	}
 	automaticCrossmountVal := ""
 	automaticCrossmountVal, runAutomaticCrossmountTest = os.LookupEnv(envVarAutomaticCrossmount)
 	automaticCrossmountEnabled, _ = strconv.ParseBool(automaticCrossmountVal)